First report of gummy stem blight caused by Didymella bryoniae on muskmelon (Cucumis melo) in India

Since 1999, severe losses have been observed on seedlings of muskmelon (Cucumis melo) in four commercial fields in the Doddaballapura, Nanjangud, Malvalli and Arasikere regions of Karnataka, India. Losses occurred during the months of October and November. Disease symptoms included stem necrosis with an exudation of gummy material, angular water-soaked lesions on the leaves and rotten fruits (Fig. 1a - c). The presence of black pycnidia was observed on the stems, leaves and fruits. A fungus was isolated from diseased stems (Fig. 1d), leaves and seeds (Fig. 1e), by plating surface sterilised plant tissues onto potato dextrose agar (PDA) medium. On PDA, the mycelium was olivaceous green and few pycnidia were observed. White aerial mycelium was also produced. Conidia were hyaline, cylindrical with rounded ends, mostly non-septate, but a few 1- septate, and 6-11 x 3-5 µm in size (Fig. 1f). Based on the symptoms and the morphological characteristics, the fungus was identified as Didymella bryoniae (Keinath et al., 1995). The pathogen causes gummy stem blight of cucurbits such as species of Citrullus, Cucumis and Cucurbita (Keinath et al., 1995). To confirm pathogenicity of D. bryoniae, 30 day-old muskmelon plants were wounded using a syringe needle and inoculated at the collar region with a suspension of 12 x 10⁵ conidia per ml in sterile distilled water. The plants were covered with plastic bags for 2 days and kept at 23°C and 90% relative humidity, with a 12 h photoperiod. Plants were assessed 7 days after infection. Inoculated plants produced typical symptoms on stems (Fig. 1g), leaves and fruits. D. bryoniae was consistently re-isolated from infected plant parts. In contrast, the control plants did not show symptoms.

The seed-borne nature of D. bryoniae was evaluated by plating different components of the seed, such as the seed coat, cotyledons and embryo, using the Standard Blotter method (ISTA, 1999) and PDA. Results clearly indicated that all parts of the seed were infected with the D. bryoniae. Subsequently, infected seeds were sown in the field and the percentage of natural transmission of the disease was recorded (41 %). D. bryoniae was re-isolated from the harvested seeds. Although the disease was previously noticed on fruits of Citrullus vulgaris var fistulosus and on leaves of Sechium edule and Praecitrullus fistulosus (Tinda) in India (Sohi & Om-Prakash, 1972), this is the first confirmed report of the pathogen on C. melo in this country. The disease incidence has increased from 13% in 1999 to 21% in 2003. Consequently, disease monitoring and management measures need to be taken.